| dc.description.abstract | We report the synthesis of monodispersed PDMS microspheres of tuneable modulus using a droplet-based microfluidics approach. Nevertheless, polymer microspheres can be used as a model representative particle to mimic the mechanical properties of RBCs (red blood cells), vesicles, and circulating tumor cells. To date, few experimental works have been reported in the literature. Reports on the microfluidic production of drops of controlled sizes from PDMS are rare because they confront difficulties in controlling droplet sizes in the dripping regime, mainly because of large viscosity. In addition, processing the highly viscous solutions into large quantities of drops of controlled sizes is difficult because the rate at which the fluid flow transitions from the dripping into the jetting regime increases with the increasing viscosity of the inner fluid. We built a PDMS-based microfluidic device using soft lithography and a glass-based microfluidic device to generate monodispersed viscous PDMS monomer droplets. The performance of each device was evaluated based on the feature size, solvent compatibility, adhesion strength and blockage problem, fabrication costs, reproducibility, rapid prototyping, and benefits in terms of scaling up for the commercialization of the product in the market. We show experimentally that glass-based coaxial devices are the most suitable for synthesizing polymer microparticles. We developed a universal flow map based on the dispersed and continuous phase fluid capillary numbers. We identified various flow regimes, such as squeezing, dripping, and jetting regimes in the flow. Further, the effect of various parameters such as viscosity ratio, capillary number of the dispersed phase, and continuous phase are studied to optimize the drop size and breakup frequency in the dripping regime. A suitable correlation was developed using the least square fitting method. Further, we measured the relaxation response of the single PDMS microsphere during the micro indentation tests. We estimated the elastic and loss modulus by fitting the relaxation data with viscoelastic models. The viscoelastic properties of the PDMS microspheres were controlled by optimizing the PDMS crosslinker concentration and curing time in the reaction mixture.
The rheological behavior of soft and hard spheres in suspension may be successful at low and intermediate volume fractions but fail at a higher concentration where the soft particles undergo deformation in shear flow. The dense suspension of rigid spheres exhibits discontinuous shear thickening transition in shear flow. This phenomenon's microscopic origin is now believed to be associated with the competition between hydrodynamic lubrication force and frictional contact between particles. Towards this, we studied various suspensions with varied parameters such as particle phase volume fraction, number of shearing cycles, solvent viscosity, and concentration of soft particles to investigate their effect on the mechanism for discontinuous shear thickening for dense suspensions in shear flow. We represent two scenarios representing the evolution of the discontinuous shear thinning transition in shear flows. One is with the rise in volume fraction, and the other is with the increase in the number of shearing cycles in shear flow. We showed that the effective transition of flow from Newtonian to continuous shear thickening and further to discontinuous shear thickening can be obtained by varying the particle phase volume fractions and number of shearing cycles and in shear flow. We also show that the discontinuous shear thickening (DST) transition can be fine-tuned by either increasing the suspending fluid viscosity, thus delaying the onset to a higher shear rate, or by adding a small percentage of non-Brownian soft particles to the suspension mixture in shear flow. The Rheogram results identify the flow transition at a critical volume fraction, shearing cycle, viscosity, concentration of soft particles, critical shear stress, and the shear rate in shear flow. The critical shear stress for the onset of the shear thickening ranges between 0.3 and 1.5 Pa, and the discontinuous shear thickening transition ranges from 200 Pa to 350 Pa. The critical packing faction at which viscosity divergence occurs is 0.43. The critical viscosity of suspending fluid above which the DST weakens to CST (Continuous Shear Thickening) for suspension in shear flow ranges from 0.025 Pas to 0.075 Pas. | en_US |
